The technology of surface acoustic waves has found many applications in the electronics and radiofrequency field. Because the wavelengths of acoustic waves are typically 105 times shorter than those of electromagnetic waves of the same frequency, this technology is particularly beneficial in applications for which miniaturization is necessary or desirable. This is the case, for example, in telephony, in which the small dimensions as well as the weight of SAW filters make components thereof highly advantageous compared with other technologies.
In general, the desirable characteristics of SAW filters are low insertion losses, a good form factor and very good rejection around the passband. Among SAW filters reputed to have such characteristics, mention may especially be made of what are called “ladder” SAW filters, two examples of which are shown in FIGS. 1A and 1B. These filters comprise SAW elements (labeled IE in the figures), for example SAW resonators of known type, that are electrically connected in cascade. The operation of these filters is described, for example, in “Advances in surface acoustic wave technology, systems and applications”, Vol. 2, pp. 67-72 (edited by C. W. Ruppel and A. Fjedly). These elements are acoustically independent and are conventionally modeled as electrical impedance elements, the admittance of which has, around a central frequency, a maximum and a minimum at frequencies called the resonant frequency and the antiresonant frequency, respectively. In what follows, these elements will simply be called “impedance elements”, which term may refer to any type of SAW element (“IDT” transducer with interdigitated electrodes, SAW resonator, or the like) used at least partly for its electrical impedance properties. In practice, these filters are conventionally produced by depositing various impedance elements on the same substrate, these elements being electrically and acoustically independent SAW resonators. To do this, in general each resonator has a specific acoustic channel. The possibility of grouping impedance elements in one and the same acoustic channel has been described (see U.S. Pat. No. 5,682,126) so as to reduce the footprint on the substrate while maintaining their acoustic independence. The elements thus grouped must be separated sufficiently from the acoustic standpoint in order not to mutually interfere.
The SAW filters shown in FIGS. 1A and 1B represent, respectively, a “nondifferential”-type arrangement (with a ground connection) and a symmetrical “differential”-type arrangement in which the useful signal is conveyed in the difference existing between the two electrical components (denoted + and −) that constitute the input (IN) or the output (OUT) of the filter. This arrangement comprises a network with symmetrical branches, the symmetry being defined with respect to an axis denoted (Δ) in FIG. 1B, lying parallel to the two branches denoted [AC] and [BD] and between them, the branch [AC] connecting the + component of the input to the + component of the output, and the branch [BD] connecting the − component of the input to the − component of the output. There is an increasing tendency in electronics for this type of arrangement with differential inputs/outputs to be preferred, this offering many advantages in integrated-circuit technology, especially greater efficiency from the standpoint of amplification and much greater noise immunity compared with that of nondifferential arrangements.
Of course, in a radio frequency-type system for example, the input point is an antenna, the signal of which is conventionally referenced with respect to ground. A filter designed for this type of application may then include a transformer-type stage using a SAW filter of the CRF/DMS (Coupled Resonator Filter/Double Mode Structure) type which, in addition to filtering, converts a nondifferential input into a differential input that is in turn applied to the input of a symmetrical array of impedance elements with differential inputs/outputs, as shown for example in FIG. 1B. A CRF/DMS-type stage is, for example, disclosed in patent application EP 0 810 727 in the name of Fujitsu.